The major theme of this work is to characterize the functional and structural plasticity of synapses formed by the primary vestibular fibers on vestibular sensory neurons in the brainstem during development and to characterize the properties of these vestibular neurons after vestibular neurectomy. All studies will be performed on slice preparations of the tangential vestibular nucleus (TN), part of the chick lateral vestibular nucleus. The largest diameter primary vestibular fibers, or colossal fibers, form large axosomatic endings-the spoon endings- on the TN principal cells. The work is focused on two ages: l) embryonic days 15/16, when gap junctions join chemical synapses to form "mixed synapses" at the spoon endings, and 2) newborns, when the gap junctions are functional at the spoon endings. The techniques used will include intracellular current and voltage clamp, intracellular dye injection, electron microscopy, and immunocytochemistry. The specific aims are to: characterize the developmental appearance of functional gap junctions at spoon synapses; define the vestibular synaptic currents generated at vestibular sensory neurons; determine the plasticity of vestibular synapses in response to tetanus and pharmacologic agents; study the membrane and synaptic properties of neurons in the ipsi- and contralateral TN after unilateral vestibular neurectomy; inject dyes intracellularly to study transient electrical coupling between vestibular neurons and determine whether TN axons project to motor neuron pools in a pattern reflecting inputs from primary vestibular fibers; and identify immunocytochemically gap junction connexins, glutamate receptors and GABA- containing terminals in the developing TN. As we learn more about the factors influencing vestibular synaptic change, we can apply this to enhance or promote new synapse formation to replace central synapses lost to disease, injury or aging. Second, it is important to study synaptic transmission so that the action of drugs on central vestibular synapses can be clarified. Third, this work will add new information on vestibular anatomy which is important to devise models on vestibular system function. Finally, we will apply multiple approaches to analyze the function of neuronal gap junctions. As gap junctions are known to exist in almost every major CNS pathway, it is meaningful to determine the specific roles that gap junctions play in the genesis and function of the central neural circuitry.